1-Butene, as well as other C4 hydrocarbons (such as isobutene and 2-butenes) are obtained in large amounts from technical C4 cuts, for example the C4 cut from steamcrackers or FCC units. These C4 cuts comprise butadiene, the monoolefins isobutene and 1-butene, the two 2-butenes, and also the saturated hydrocarbons isobutane and n-butane. Owing to the low boiling point differences of the ingredients and their low separation factors, a distillative workup is difficult and uneconomic. Linear butenes and other products are therefore usually obtained by a combination of chemical reactions and physical separating operations.
The first step in obtaining ethyl tert-butyl ether, which is common to all workup variants, is frequently the removal of most of the butadiene. The butadiene is removed by extraction, extractive distillation, or it is hydrogenated selectively to linear butenes down to a residual concentration of approximately 2000 ppm by mass. What remains, after any of these operations, is a hydrocarbon mixture (so-called raffinate I or hydrogenated crack-C4). The raffinate I comprises the saturated hydrocarbons n-butane and isobutane, the olefins isobutene, 1-butene and the cis and trans 2-butenes.
Alkyl tert-butyl ether(s) (ATBE), in particular ethyl tert-butyl ether (ETBE) and methyl tert-butyl ether (MTBE), can be obtained by reacting the isobutene present in the raffinate I with an alcohol. After the conversion of the isobutene and removal of the alkyl tert-butyl ether, what remains is a hydrocarbon mixture (raffinate II). The raffinate II comprises the linear butenes and the saturated hydrocarbons isobutane and n-butane. This components of raffinate II can be separated further by distillation, for example into isobutane and 1-butene and a mixture of two 2-butenes and n-butane. In further distillation steps, 1-butene which contains only small amounts of isobutene can be obtained in high purity from the 1-butenic fraction. Highly pure 1-butene is desirable because 1-butene is used to a large degree as a comonomer in ethylene polymerization, where isobutene contaminations are undesired. Typical specifications of 1-butene therefore restrict the content of isobutene in the 1-butene to below 2000 ppm. Processes for preparing ATBE from C4 hydrocarbon fractions comprising isobutene therefore have high economic viability, especially when the ATBE can be prepared without large losses of 1-butene.
For the reaction of isobutene with alcohols, for example methanol or ethanol, to give the corresponding tertiary butyl ethers, several process technology variants have been developed. The technique of reactive distillation has been found to be particularly useful for achieving high isobutene conversions.
Industrially, the most important process is the reaction of isobutene with methanol to give methyl tert-butyl ether (MTBE). Methyl tert-butyl ether is used mainly as a fuel additive. Because of the ever greater availability of ethanol from renewable raw materials, the demand for ETBE as a fuel additive is also increasing.
EP 0 048 893 describes a process for coproducing isobutene oligomers and alkyl tert-butyl ether (ATBE), from C4 cuts, in one reactor. The process employs a catalyst. The catalyst is an acidic ion exchange resin which has been partly modified with metals of the seventh and eighth transition Groups of the Periodic Table of the Elements. After the catalytic reaction, the isobutene oligomers and alkl tert butyl ether, and the unconverted C4 hydrocarbons, are separated by distillation. In this process, approximately 8% of the linear butenes are lost by oligomerization. The loss of 1-butene is 7%. However, the main disadvantage of this process is that full isobutene conversion is not achieved, resulting in an isobutene content, in the distilled C4 hydrocarbon fraction, that is too high to obtain on-spec 1-butene therefrom.
DE 25 21 964 describes a two-stage process for preparing alkyl tert-butyl ethers. In the first stage, isobutene is reacted with alcohol for form an ether, and the ether is removed from the product mixture of the first stage. The remaining residue of the product mixture is conducted into a second reaction stage in which remaining isobutene is again reacted with alcohol.
U.S. Pat. No. 4,797,133 describes a process wherein initially, the isobutene content of the starting hydrocarbon stream is reduced, for example by reacting isobutene to give methyl tert-butyl ether (MTBE), and the methyl tert-butyl ether is then removed. The remaining residue of the starting hydrocarbon stream is subsequently fed to an etherification stage in which the remaining isobutene is converted.
EP 0 071 032 likewise describes a two-stage process for preparing ETBE in which the ETBE formed in the first stage is removed from the reaction mixture between the first and second stage.
A disadvantage of all of these processes is the large amounts of material which have to be conducted into the second reaction step.